We often take for granted the very special environment in which we live. The large mass of the Earth is able to hold together a complex atmosphere of nitrogen, oxygen, carbon dioxide and water vapor over its surface so we can breathe and live comfortably, free from harmful radiation and most incoming meteoroids.

When astronauts go up into space, they have to bring a similar environment with them in order to survive -- without it, their bodies would suffer from a complete lack of air and pressure. Space shuttles and space stations are filled with the same kind of air found at sea level, and except for the effects of microgravity, crew members live and breathe quite normally. If astronauts ever have to leave the confines of a spacecraft, though, they'll have to take part in the highly specialized activity known as spacewalking.

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Despite being multibillion-dollar projects, space stations and space shuttles aren't perfect. They're complex systems with lots of intricate pieces, and if something goes wrong, it could cause major damage, compromising the mission, the expensive technology and, most importantly, the people on board.

So when something needs to be repaired, removed or added and robotics just won't do the trick, specially trained astronauts have to conduct spacewalks to take care of the problem with human finesse. The events can last for very long periods of time, and, like cave diving underwater, require a great deal of training, equipment and technology to pull off safely.

So what does it take to spacewalk? What keeps a spacewalker safe in the vacuum of space? What if you have to go to the bathroom out there? To learn about spacewalks, read the next page.

Extravehicular Activity and the Dangers of Space

If astronaut Dave Williams didn't have his space suit on while making repairs to the International Space Station, much of his body would experience significant, dangerous swelling.

NASA employs two types of astronauts: pilot astronauts and mission specialist astronauts. While pilots are there to fly shuttles and command missions, the mission specialists are the ones who maintain the spacecraft, conduct experiments and perform any necessary spacewalks.

Although spacewalk is the common name most people use, there's actually an official term: extravehicular activity (EVA). An EVA can actually have several definitions, and it all depends on the country performing the spacewalk. The Russians, for example, define an EVA as any time a cosmonaut spends time in a vacuum in a space suit. Russian and Soviet spacecraft have specialized airlocks for spacewalks; even though a cosmonaut might not be completely out of the ship, he or she is still under the protection of a space suit. The American definition, on the other hand, has at least the astronaut's head outside of the spacecraft.

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Either way, an EVA happens when an astronaut leaves the protective environment of a spacecraft and enters outer space, the area that lies outside of the Earth's atmosphere. The void of space is commonly referred to as a vacuum, which means there's a near or total lack of gas molecules. The gravitational attraction of the planets and stars pulls most gas toward them, leaving the areas in between practically empty.

Luckily for us, things are much more hospitable on Earth. The atmosphere, mainly a mixture of nitrogen, oxygen and hydrogen gases, exerts a pressure on our bodies that we're used to -- about 101 kilopascals at sea level.

In space, however, there's a complete lack of molecules, which means there's also a complete lack of pressure. This is one of the major reasons it's dangerous for astronauts to go into space without a proper space suit. If you were to exit a spacecraft just wearing jeans and a T-shirt, the air inside your lungs would quickly rush out of your body because of the lack of air pressure. Gases in body fluids would expand, pushing your insides around in grisly ways, and your skin would inflate like a balloon.

There are several other hazards that come with venturing out into space unprotected. Temperatures outside of a spacecraft fluctuate wildly depending on where you are. Sunlit objects above the Earth's atmosphere can reach more than 248 degrees Fahrenheit (120 degrees Celsius), while shaded ones can reach the opposite extreme -- lower than negative 212 degrees F (100 degrees C). On top of this, the radiation from the sun, ultraviolet radiation and tiny meteoroids speeding through space pose potential dangers.

Fortunately, a space suit keeps astronauts alive in the vacuum of space while providing enough mobility to move around and accomplish tasks. To learn about the space suit an astronaut uses for spacewalks, read the next page.

The First Spacewalks

The first human in history to conduct a spacewalk was Aleksei Leonov of Russia, during the Voskhod II program. After launching into orbit on March 18, 1965, Leonov exited the spacecraft and took part in extravehicular activity for about 12 minutes. The historic event almost ended in disaster, though, when his space suit inflated while he was outside and prevented him from re-entering the airlock. Opening a valve and risking decompression sickness -- or "the bends," the illness divers experience after surfacing too quickly -- Leonov released as much air from the suit as possible and made his way back inside.

The first American to spacewalk was Edward White during the Gemini 4 mission on June 3, 1965. White tested a Hand-Held Maneuvering Unit, a small jet held by astronauts to propel them around during spacewalks.

Spacewalks and Space Suits

Space suit for spacewalks

2008 HowStuffWorks

The current space suit design used by astronauts aboard the International Space Station is a result of more than 60 years of innovation. Space suits have their own technical term used by NASA officials: extravehicular mobility unit (EMU). Astronauts actually refer to a space suit as a minivehicle or vessel, not just a suit -- it does, in fact, weigh over 240 pounds (108.9 kilograms) on Earth.

This first thing you'd notice after putting on a space suit before going on a spacewalk is that the suit is very bulky. Although NASA is constantly working to make suits slimmer and more ergonomic, they have to be quite hefty by nature because that provides the necessary pressure to survive in a vacuum. An EMU actually has an impressive total of 14 layers that combine to offer heating, cooling and a pressurized system. The first three consist of the liquid-cooling-and-ventilation garment (LCVG), which is essentially like long underwear made of nylon and spandex. It's laced with tiny plastic tubing for cooling purposes and gas connectors for ventilation. Above that is a pressure garment bladder, an inflated layer of urethane-coated nylon that provides pressure, followed by a pressure restraint called Dacron. The final nine layers consist of the thermal micrometeoroid garment, which is made of a multilayered aluminized fabric called Mylar that protects astronauts from penetrating objects. This doesn't even count the Maximum Absorption Garment (MAG), which is the very first piece of clothing astronauts put on. It's essentially an adult diaper that collects an astronaut's urine -- an EVA may last for as long as seven hours, and you never know when nature will call in the void of space.

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There are, along with the LCVG and MAG, 18 separate pieces to a space suit, including the Primary Life-Support System (PLSS), a backpack unit that provides astronauts with oxygen, carbon-dioxide removal, electrical power, a water-cooling system and communication, a Displays and Control Module (DCM) and a helmet and visor. Putting on a space suit, despite its many parts, is actually a fairly quick process -- astronauts can put everything together in about 15 minutes. The process a spacewalker must undertake before leaving the protection of a spacecraft actually takes longer. To learn about spacewalk procedure and preparation, read the next page.

Origins of the Space Suit

The development of space suits didn't directly begin with mankind's interest in getting outside of the Earth's atmosphere. Instead, early aviation pioneers from the '30s attempting to break altitude records were trying to figure out how to fly higher. Unfortunately, the side effect of flying extremely high in the Earth's atmosphere is dealing with the thin air and low pressure, and the experience is dangerously similar to being in the vacuum of space. An American aviator named Wiley Post was the man who eventually devised the protection pilots needed. Since pressurized cabins like the ones on modern airplanes weren't invented, Post developed a series of pressurized suits that protected him from low pressures, greatly influencing future development of the space suit.

Spacewalk Procedure and Preparation

U.S. Space Shuttle Atlantis mission specialist Pat Forrester outside of the Quest airlock of the International Space Station.

The process leading up to a spacewalk takes almost an entire day, and it's not because of the many items that make up an astronaut's suit. The main reason for this is because astronauts need time to go through decompression, the same procedure cave divers use when returning from the depths of the ocean to the surface of the water.

To allow maximum mobility and maximum protection from the lack of pressure in space, a space suit is pressurized at 29.6 kilopascals during a spacewalk, about one third of the pressure experienced by the crew inside the spacecraft. Astronauts also have to breathe in pure oxygen, because the amount of oxygen in air at such a low pressure isn't enough.

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Now, if an astronaut simply donned a space suit in 15 minutes and promptly exited an airlock, he or she would go through decompression sickness, or "the bends" -- the same thing scuba divers experience if they're exposed to a rapid drop in external pressure by ascending too quickly. The bends causes expanded nitrogen gas bubbles in the bloodstream to escape too quickly, and joint pain, dizziness, cramps, paralysis and even death can follow.

The airlock of the International Space Station.

2008 HowStuffWorks

To avoid this, the entire cabin undergoes decompression from the normal 101 kilopascals to 70.3 kilopascals and a slight increase in oxygen at least 24 hours before the spacewalk begins. One hour before a spacewalk, astronauts wear a mask and prebreathe pure oxygen, which will prepare them for breathing 100 percent oxygen.

The majority of this takes place inside an airlock. The airlock is a small cylindrical area, only 5.25 feet (1.6 meters) in diameter and 6.9 feet (2.1 meters) high, located on a spacecraft's mid-deck. A hatch from the mid-deck leads to the airlock, and the airlock is connected by another hatch to an unpressurized payload bay, which leads to outer space. After an astronaut dons the full space suit inside the airlock and shuts himself off completely from the outside atmosphere, the inner hatch of the airlock is sealed and the pressure inside is gradually decreased. Once the area reaches the appropriate pressure, the astronaut pulls himself through the outer airlock hatch into the payload bay and finally begins the EVA.

To learn about what it's like to work in space, read the next page.

Types of Spacewalks and Necessary Equipment

Once the hatch to the exterior airlock is open and the astronaut pokes his or her head out into the void, several tools give a variety of support to a spacewalker. One of the most important pieces of equipment makes sure astronauts don't fly away from the spacecraft and either burn up in the Earth's atmosphere or venture off into the deep nothing of space. A crewmember safety tether, the equivalent of a simple rope, keeps an astronaut on a spacewalk completely attached to the shuttle or station.

Safety tethers are 55 feet (16.8 meters) long, ensuring the necessary mobility, and they're attached at all times to a waist tether by a large hook. Extra tethers on the wrists keep hand tools from flying away. Slidewires, handrails and footholds are also added to spacecraft to make it easier for spacewalkers to move around and perform simple jobs like turning a nut with a wrench.

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Astronaut Bruce McCandless II using a manned maneuvering unit (MMU) outside of the Space Shuttle Challenger. This was the first untethered spacewalk in history.

Spacewalkers can also use manned maneuvering units (MMUs), larger propulsive vehicles attached to space suits, for extra mobility and more complicated tasks. Astronauts always bring along simplified aid for EVA rescue (SAFER), which are smaller, emergency MMU systems that are attached to the space suit's backpack.

If an astronaut gets thirsty during a spacewalk, suits are equipped with an in-suit drink bag (ISD), which is just a plastic water-filled bag with a drinking tube that's attached by Velcro to the front interior of the suit. Astronauts just turn their head toward the tube and take a sip. Although suits used to attach fruit bars near astronauts' necks, most spacewalkers today prefer to eat before they exit spacecraft.

There are three basic types of spacewalks:

A scheduled spacewalk is any EVA that's part of the plan. Many space shuttles will fly up to space stations with several goals in mind, and one of those goals may be to replace some faulty wires or tighten up any loose screws on a spacecraft. A spacewalk will, of course, be necessary to complete this goal, so it will be an important part of a mission's schedule.

An unscheduled spacewalk is one that wasn't part of the mission's flight plan but is necessary for its completion.

A contingency spacewalk is the same as an unscheduled one, but it must be done to ensure the safe return of the crew and spacecraft.

Extravehicular activities are also divided up by level of complexity. A simple EVA uses few tools or mobility techniques, and astronauts don't need a lot of training or briefing before these types of spacewalks. An intermediate EVA might need special tools and equipment, and more in-depth training beforehand is probably necessary. A complex EVA requires more complicated technology and will most likely be difficult and dangerous to complete, and astronauts will need to have had a great deal of training. The type of training an astronaut usually receives, believe it or not, takes place in pools. Spacewalking is often compared to scuba diving -- the material used for space suits like neoprene and nylon, for example, is also used for scuba suits. After as many as 25 to 30 hours of practice in pools, astronauts have in fact compared the sensation of floating underwater to floating in space [source: NASA].

To learn more about interstellar travel and space exploration, see the articles on the next page.